Technical challenges in the design of integrated circuits, such as monolithic microwave integrated circuit (MMIC) modules, include minimizing return loss, especially when operating over a wide bandwidth and at high frequencies. Moreover, excessive heat generation in these radio frequency (RF) applications can be a limiting factor in both the performance and longevity of a device. In fact, achieving suitable performance in an MMIC module design with a return loss performance of 15 dB or better over a wide bandwidth (i.e. greater than an octave bandwidth) at high frequency (e.g. Ku-band and higher), while retaining a thermal path to channel heat from the MMIC device, has proven to be impossible using ribbon or bond-wire techniques to connect an MMIC to a connectorized substrate or package housing.
More specifically, a common technique for designing an active MMIC module for a transmit/receive or multiple input/output port switch matrix application is to bond an MMIC device with its active side up onto a module substrate or package. Once bonded, direct current (DC) and RF interconnections between the MMIC and module package are made using conductive ribbons or bond wires. An exemplary package resulting from this process is shown in FIG. 1. Package 10 includes an MMIC 12 mounted to a cold plate 14. Electrical connections (e.g. RF connections) are made by bond wires 16 arranged between a top active surface of MMIC 12 and, for example, RF ports or terminals 17 of package 10. In this implementation, the use of ribbons or bond wires introduce randomly-variable inductances which limit return loss performance over a wide bandwidth at high frequency.
In addition, matching and tuning techniques, such as matching networks 18, are typically needed to attempt to account for the inductances created by the wire bonding. However, these networks typically add additional insertion loss, in addition to requiring significant surface area that increases overall package size and complexity. Further still, as the inductances are often varying over frequency, it is difficult to design a matching network which remains effective over a wide bandwidth at high frequency.
In order to minimize these inductance variations, alternative packaging methods have been developed. For example, “flip chip” arrangements include MMIC devices mounted upside-down, using solder bumps to make the electrical connections between the active surface of the MMIC to the module package. An exemplary arrangement is illustrated in FIG. 2, wherein package 20 includes an inverted MMIC 22 which is mounted to a corresponding pad formed on surface 24 of package 20 via solder bumps 28. However, such a soldered configuration results in the formation of a gap 26, wherein MMIC 22 is suspended over packaging mounting surface 24. This gap may comprise an air gap, or may be filled with an adhesive or epoxy. The implementation of solder bumps yields excellent return loss performance (15 dB or greater) over a wide bandwidth at high frequency. However, these arrangements lack a viable thermal path to cool the MMIC, as there is no direct contact to any metallic surface (e.g. to a cold plate). Moreover, it is quite difficult to align all DC and RF pads to corresponding solder bumps on a complex active MMIC, including those with multiple input/output ports.
Alternative structures and methods of mounting active semiconductor devices are desired.